The invention relates to reversible liquid/solid phase change compositions. More particularly, the invention resides in phase change compositions comprising a mixture of hydrated calcium chloride and a potassium salt whose anion forms a substantially less soluble salt with calcium.
Phase change materials (PCM's) in which the heat of fusion of various hydrated salt compositions is employed are well known in the literature. In the ASHRAE Journal of September, 1974, entitled SOLAR ENERGY STORAGE, Dr. M. Telkes evaluated the thermal, physical and other pertinent properties of PCM's on the basis of economics, applicability, corrosion, toxicity and availability for large scale installations. Among the materials evaluated were various salt hydrates and their eutectics including CaCl.sub.2.6H.sub.2 O which undergoes several phase transitions to materials of different crystal structure, i.e. CaCl.sub.2.6H.sub.2 O to CaCl.sub.2.4H.sub.2 O+2H.sub.2 O at 29.degree. C.
When heated to a temperature of above 33.degree. C., the salt CaCl.sub.2.6H.sub.2 O dissolves completely in its water of crystallization. When cooled, formation of four different crystal forms is possible, i.e., CaCl.sub.2.6H.sub.2 O and three forms of CaCl.sub.2.4H.sub.2 O. If any of the 4H.sub.2 O crystals form, the heat of fusion is much less than 46 cal/gm (CaCl.sub.2.6H.sub.2 O in substantially pure form undergoes a liquid/solid phase transition at about 30.degree. C. releasing or alternately absorbing about 46 calories of heat per gram). Despite the relatively low cost of CaCl.sub.2, the formation of its four different crystal forms was deemed to be disadvantageous.
Carlsson et al., in Swedish Pat. No. 78,01037-8 (Publication No. 410,004), claim a method for suppressing the tetrahydrate formation during repeated melting and crystallization of a system based on CaCl.sub.2.6H.sub.2 O. Carlsson et al., determined that in solutions in the concentration range of from 48 to 53 weight percent CaCl.sub.2, using CaCl.sub.2.6H.sub.2 O of highest purity, the crystallization temperatures for CaCl.sub.2.6H.sub.2 O and CaCl.sub.2.4H.sub.2 O were such that the solution was incongruently melting and that CaCl.sub.2.4H.sub.2 O crystallized and precipitated out of the solution thus losing its heat storage capacity. By using a solution of the same concentration from CaCl.sub.2 of technical grade (Road Salt), containing NaCl and KCl as impurities, the solubility of the tetrahydrate decreased and that of the hexahydrate increased. However, on repeated melting and crystallization, the precipitation becomes significant and the system again loses its heat storage capacity. Thus, the conclusion can be drawn that the use of technical grade CaCl.sub.2 (Road Salt) results in a poorer performance due to a relative increase in tetrahydrate formation as compared to a system based on high purity CaCl.sub.2.
Carlsson et al., also discovered that the addition of one or more compounds, including about 2 weight percent SrCl.sub.2.6H.sub.2 O, increased the solubility of the tetrahydrate and suppressed tetrahydrate formation on repeated melting and crystallization. The amount of addition was found to be dependent upon the amount of impurities present in the system which, in an example using Road Salt was determined to be 2.2 weight percent.
The relative amounts of each impurity in the technical grade salt (Road Salt) was not determined nor was it held to be important to the outcome of the tests conducted. In fact, the use of Road Salt was found to be less desirable from the standpoint of tetrahydrate formation compared to CaCl.sub.2 of high purity. Neither was there any recognition by Carlsson et al., that impurities of NaCl and KCl in the composition could be beneficial in reducing tetrahydrate crystal formation in such phase change compositions.
In our copending U.S. application Ser. No. 364,159, filed Mar. 31, 1982, it was shown that the addition of KCl to CaCl.sub.2.6H.sub.2 O greatly reduces the possibility of forming the undesired CaCl.sub.2.4H.sub.2 O crystalline phase during the retrieval of the stored heat on freezing of the phase change composition. The present invention now surprisingly shows that the desirable effect of reducing the formation of the undesired CaCl.sub.2.4H.sub.2 O crystalline phase can also be achieved with salts other than the chloride, i.e., KCl. In particular, the reduction of the formation of the CaCl.sub.2.4H.sub.2 O crystalline phase during the retrieval of stored heat on freezing of the hydrated CaCl.sub.2 composition can be achieved by the addition of a potassium salt in which the anion of the potassium salt employed forms a sparingly soluble calcium salt, i.e., a calcium salt which is substantially insoluble.
Our copending U.S. application Ser. No. 364,159 also shows that the addition of NaCl and/or SrCl.sub.2 augments the beneficial effect produced by the addition of KCl to the hydrated CaCl.sub.2 composition to thereby obtain a composition which is an effectively congruently melting composition. It has now been discovered that other sodium and/or strontium salts can also be employed with the same beneficial results.
In some applications, for example, it is preferable to use potassium, sodium or strontium salts or mixtures of such salts, other than salts in which the anion is chloride, to increase the pH of the composition and to thereby reduce the natural acidity of aqueous CaCl.sub.2. In systems, i.e., phase change compositions in which the anions of potassium, sodium and strontium salts form a substantially less soluble salt with calcium, the compatibility of the heat storage composition in metal containers is substantially improved.
Heat storage compositions are ideally packaged in individual encapsulating means for use in conjunction with solar heating systems. Exemplary of suitable known encapsulating means for the heat storage compositions herein described are water impervious films or foils of plastic/metal laminates. Closed cell plastic foams have also been suggested in which the PCM may be encapsulated within the cells of the foam structure as illustrated in, for example, U.S. Pat. No. 4,003,426. Other useful encapsulating means are concrete, metal or plastic containers, pipes, and the like.